1. Field of the Invention
The invention relates to an automatic transmission control system, especially a hydraulic control system in which governor pressure is supplied as an automobile speed signal to a shift valve so as to initiate a shift change of the transmission.
2. Description of the Related Art
An automatic transmission control element is shown in Japanese Patent application 55-36633 and, for example, an overdrive shift valve 1 is shown in FIG. 4. A sleeve 2 is slidable in a valve body, a return spring 3 is disposed between the sleeve 2 and the valve body, a throttle pressure Pth is supplied from an oil passage L1 adjacent a land 4, a governor pressure Pgo is supplied from an oil passage L2 to urge a land 5 against the force of the spring 3 and the throttle pressure Pth. When the pressures satisfy the following formula (1), the shift valve 1 is changed to an upshift position. EQU Pgo.times.A1.gtoreq.Pth.times.A2+Fs-Pth.times.A3 (1)
wherein
A1: area of the spool 5 acted on by the governor pressure Pgo PA1 Fs: force of the return spring 3 PA1 A2: area of the spool 4 acted on by the throttle pressure Pth PA1 A3 area of the spool 2 acted on by the throttle pressure Pth A upshift changing boundary of the shift valve 1 is fixed as shown in FIG. 7. That is, at the area .THETA..sub.2 .about.0 corresponding to the degree of the throttle opening, A2=A3 and Pgo.times.A1=Fs=const. Therefore, automobile speed at the changing point is V2 (equal to a constant, shown as line o in FIG. 7) irrespective of the changing of the throttle pressure Pth. At the area over .THETA..sub.2, if Pth.times.A3 is equal to FS at the point M, the formula (1) shows Pgo.times.A1.gtoreq.Pth.times.A2 and a shift of the transmission occurs. In this area, the spool 2 is urged by the throttle pressure Pth to an upper position and the governor pressure Pgo is equal to mrw.sup.2 /Ago, therefore, the changing boundary shows a secondary curve B corresponding to automobile speed.
In a known 2-3 shift valve shown in FIG. 8, a 1.fwdarw.2 shift point is fixed and the characteristics of the throttle pressure Pth and governor pressure Pgo are determined in order to select a performance driving mode or a fuel economy mode. The characteristics or modes are shown as in FIGS. 5 and 6. When the 2.fwdarw.3 shift point is fixed in the same way, only Fs, A1, A2 in FIG. 8 are fixed, because the throttle pressure Pth and governor pressure Pgo have been determined. Therefore the shift point is inevitably decided as shown in full line in FIG. 9. That is, the 2.fwdarw.3 shift point crosses the 1-2 shift point at the area of a reduced degree of throttle opening. In FIG. 9, the 2.fwdarw.3 shift point is moved to the right by the changing of the ratio of A1 to A2 of the 2.fwdarw.3 shift valve and Fs. However, the shift point shown with a dotted line which has a point of inflection at V2 is not obtained, because the characteristics of the throttle pressure Pth and the governor pressure Pgo have been determined by the full lines shown in FIGS. 5 and 6.
The determination of the shift point of the automatic transmission has to correspond with the operating characteristic of the engine or the dimensions of the automobile. In the shift valve 1 in FIG. 4, however, the shift point is constant at the automobile speed V2 in the area below a degree of throttle .THETA.2. Therefore, the shift point does not change corresponding to the degree of throttle opening and the automobile speed shown with UP1 or UP2 in FIG. 3 and the performance driving mode or the economy driving mode often is not satisfactory. If the shifting point shown in FIG. 3 is adapted to the shift valve 1 shown in FIG. 4, the throttle valve and the governor valve have a complicated construction in which the pressurized area or the dimensions of the spool or the valve body is changed. In the shift valve shown in FIG. 8, the 1-2 shift point and the 2-3 shift point cross each other as shown in FIG. 9. Accordingly, the shift feel or shift shock worsens or becomes more pronounced. Consequently, if the shift valve shown in FIG. 4 is used as the 2.fwdarw.3 shift valve in order to prevent crossing of the 1.fwdarw.2 changing point and the 2.fwdarw.3 changing point, it is difficult to fix the shift point in accordance with the shift change condition. If the shift valve in FIG. 8 has a changing point at the automobile speed V2 as shown with the dotted line in FIG. 9, the governor pressure characteristic has to be shown the three steps having a point of inflection at the V2.